Organic thin film transistor(s) and method(s) for fabricating the same

ABSTRACT

Example embodiments of the present invention for fabricating an organic thin film transistor including a substrate, a gate electrode, a gate insulating layer, metal oxide source/drain electrodes and an organic semiconductor layer wherein the metal oxide source/drain electrodes are surface-treated with a self-assembled monolayer (SAM) forming compound containing a sulfonic acid group. According to example embodiments of the present invention, the surface of the source/drain electrodes may be modified to be more hydrophobic and/or the work function of a metal oxide constituting the source/drain electrodes may be increased to above that of an organic semiconductor material constituting the organic semiconductor layer. Organic thin film transistors fabricated according to one or more example embodiments of the present invention may exhibit higher charge carrier mobility. Also disclosed are various example devices including display devices having organic thin film transistors made by example embodiments of the present invention.

PRIORITY STATEMENT

This non-provisional application claims priority under 35 U.S.C. § 119of Korean Patent Application No. 2005-56196 filed on Jun. 28, 2005, theentire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Example embodiments of the present invention relate to an organic thinfilm transistor and to a method for fabricating the same. Moreparticularly, example embodiments of the present invention relate to amethod for fabricating an organic thin film transistor with improved(e.g., higher) charge carrier mobility. According to example embodimentsof the present invention, the organic thin film transistor may include asubstrate, a gate electrode, a gate insulating film, metal oxidesource/drain electrodes and an organic semiconductor layer. In exampleembodiments of the present invention, the metal oxide source/drainelectrodes may be surface-treated with a self-assembled monolayer (SAM)forming compound containing a sulfonic acid group so that thehydrophobic quality, charge carrier mobility and/or work function of ametal oxide constituting the source/drain electrodes may be increased.

2. Description of the Related Art

With the advent of polyacetylenes as conjugated organic polymersexhibiting semiconductor characteristics, organic semiconductors havebeen investigated as electrical and electronic materials in a widevariety of applications, e.g., functional electronic and opticaldevices. Because organic semiconductors may lend themselves to varioussynthetic processes, may be easier to mold into fibers and films, andmay exhibit superior flexibility, higher conductivity organicsemiconductors may provide a way to lower manufacturing costs.

Organic thin film transistors may have advantages in that semiconductorlayers may be formed by printing processes at ambient pressure insteadof being formed by conventional silicon processes such asplasma-enhanced chemical vapor deposition (CVD). Also, if needed, theoverall fabrication procedure may be achieved by roll-to-roll processesusing plastic substrates, which may be economically advantageous oversilicon thin film transistors. Among other possible uses, thin filmtransistors may be integrated into active displays and plastic chips (orother suitable chip material) for use in smart cards and inventory tags.

Nevertheless, organic thin film transistors may suffer from having a lowcharge carrier mobility, high driving voltage and/or high thresholdvoltage in comparison with silicon thin film transistors. But organicthin film transistors having a charge carrier mobility of about 0.6cm².V⁻¹. sec⁻¹ using, for example, pentacene in an organic thin filmtransistor may still be put to practical use. The charge carriermobility of the organic thin film transistor, however, may still beunsatisfactory. Further, there may be some disadvantages of organic thinfilm transistors because they may require a driving voltage of about 100V or more and a threshold voltage of about 50 times higher than thatrequired of silicon thin film transistors.

Sometimes when pentacene is used as the organic semiconductor materialin a bottom-contact or top-gate organic thin film transistor, thepentacene is prone to deposit on a gate insulating layer rather than onsource/drain electrodes and may have a relatively high work functioncompared to metal source/drain electrodes. Consequently, a Schottkybarrier may be formed between the source/drain electrodes and theorganic semiconductor layer, which may lower the charge carrier mobilityof the organic thin film transistor.

According to a conventional process, organic thin film transistors maybe produced by first treating the exposed surface of source/drainelectrodes with a self-assembled monolayer compound containing a thiolfunctional group before deposition of an organic semiconductor layer.However, the monolayer compound may become bound to metal surfaces,e.g., gold (Au), while remaining unbound to the surface of metal oxides,e.g., indium-tin oxide. Thus, formation of improved thin filmtransistors comprising metal oxide source/drain electrodes and anorganic semiconductor layer by such monolayer process has proved to besomewhat difficult.

SUMMARY

An example embodiment of the present invention provides a method offabricating an organic thin film transistor with one or more improvedelectrical properties. For example, higher charge carrier mobility maybe achieved by treating the surface of metal oxide source/drainelectrodes with a self-assembled monolayer forming compound containing asulfonic acid group (—SO₃H). The (—SO₃H) group is capable of stronglyadsorbing on the metal oxide of the source/drain electrodes so that themetal oxide may have a higher work function than an ordinary organicsemiconductor material.

In accordance with an example embodiment of the present invention, thereis provided a method for fabricating an organic thin film transistorincluding a substrate, a gate electrode, a gate insulating film, metaloxide source/drain electrodes and an organic semiconductor layer whereinthe metal oxide source/drain electrodes may be surface-treated (orcoated) with a self-assembled monolayer (SAM) forming compoundcontaining, for example, a sulfonic acid group.

In accordance with another example embodiment of the present invention,an organic thin film transistor may be fabricated by the above notedmethod.

In accordance with yet another example embodiment of the presentinvention, a display device using the above-noted (SAM with sulfonicacid group coating) organic thin film transistor may be made.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments the present invention may be more clearly understoodfrom the following non-limiting detailed description taken inconjunction with the accompanying drawings. FIGS. 1-2 representnon-limiting examples, embodiments and/or intermediates of the presentinvention as described herein.

FIG. 1 is a cross-sectional view schematically showing the structure ofan organic thin film transistor fabricated in accordance with an exampleembodiment of the present invention; and

FIG. 2 is a graph showing the current transfer characteristics oforganic thin film transistors fabricated pursuant to non-limitingExamples 1 to 4 of the present invention and pursuant to ComparativeExamples 1 to 4.

DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

Various example embodiments of the present invention will now bedescribed more fully with reference to the accompanying drawings inwhich some example embodiments of the invention are shown. In thedrawings, the thicknesses of layers and regions may be exaggerated forclarity.

Detailed illustrative embodiments of the present invention are disclosedherein. However, specific structural and functional details disclosedherein are merely representative for purposes of describing exampleembodiments of the present invention. This invention may, however, maybe embodied in many alternate forms and should not be construed aslimited to only the embodiments set forth herein.

Accordingly, while example embodiments of the invention are capable ofvarious modifications and alternative forms, embodiments thereof areshown by way of example in the drawings and will herein be described indetail. It should be understood, however, that there is no intent tolimit example embodiments of the invention to the particular formsdisclosed. To the contrary, example embodiments of the invention arealso intended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention. Like numbers refer to likeelements throughout the description of the figures.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of example embodiments of thepresent invention. As used herein, the term “and/or” includes any andall combinations of one or more of the associated listed items.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between” versus “directly between”, “adjacent” versus “directlyadjacent”, etc.).

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments of the invention. As used herein, the singular forms “a”,“an” and “the” are intended to include the plural forms as well, unlessthe context clearly indicates otherwise. It will be further understoodthat the terms “comprises”, “comprising”, “includes” and/or “including”,when used herein, specify the presence of stated features, integers,steps, operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof

It should also be noted that in some alternative implementations, thefunctions/acts noted may occur out of the order noted in the FIGS. Forexample, two FIGS. shown in succession may in fact be executedsubstantially concurrently or may sometimes be executed in the reverseorder, depending upon the functionality/acts involved.

An example embodiment of the present invention provides a method forfabricating an organic thin film transistor including a substrate, agate electrode, a gate insulating film, metal oxide source/drainelectrodes and an organic semiconductor layer wherein the metal oxidesource/drain electrodes are surface-treated with a self-assembledmonolayer forming compound containing, for example, a sulfonic acidgroup.

Non-limiting examples of organic thin film transistors that may befabricated (by one or more example embodiment methods of the presentinvention) include (but are not limited to) bottom-contact organic thinfilm transistors with a bottom-gate and top-gate organic thin filmtransistors. According to an example embodiment of the presentinvention, bottom-contact organic thin film transistors may befabricated by, for example, a method including: forming a gate electrodeon a substrate; forming a gate insulating film thereon; forming metaloxide source/drain electrodes on the gate insulating film; forming aself-assembled monolayer on the surface of the metal oxide source/drainelectrodes; and forming an organic semiconductor layer on the gateinsulating film and the self-assembled monolayer. According to anotherexample embodiment of the present invention, top-gate organic thin filmtransistors may be fabricated, for example, by a method including:forming metal oxide source/drain electrodes on a substrate; forming aself-assembled monolayer on the surface of the metal oxide source/drainelectrodes; forming an organic semiconductor layer between the metaloxide source and drain electrodes; forming an insulating film thereon;and forming a gate electrode on the insulating film.

Example embodiments of the present invention are described in thecontext of fabrication of a bottom-contact organic thin film transistor.However, example embodiments of the present invention may also beapplied to the fabrication of a top-gate organic thin film transistor.An example embodiment of the present invention (e.g., for fabricating abottom-contact organic thin film transistor) may include:

optionally washing a substrate to remove impurities by commontechniques, and forming a gate electrode on the substrate by, forexample, deposition such as pattern deposition.

The substrate may be made of, without limitation, glass, silicon,plastic, etc. Other suitable substrate materials may be used.

As a suitable material for the gate electrode, a metal or anelectrically conductive polymer may be used. Specific examples of gateelectrode materials include, but are not limited to, gold (Au), silver(Ag), aluminum (Al), nickel (Ni), molybdenum (Mo), tungsten (W),indium-tin oxides (ITO), polythiophenes, polyanilines, polyacetylenes,polypyrroles, polyphenylene vinylenes, and polyethylenedioxythiophene(PEDOT)/polystyrenesulfonate (PSS) mixtures. Other suitable gateelectrode materials may be used.

After forming the gate electrode(s) on the substrate, a gate insulatinglayer may be formed thereon by common techniques.

Examples of suitable materials for the gate insulating layer used inconjunction with organic thin film transistors made according to exampleembodiments of the present invention include, but are not limited to,organic materials, such as polyolefins, polyvinyls, polyacrylates,polystyrenes, polyurethanes, polyimides, polyvinylphenols andderivatives thereof, and inorganic materials, such as SiN_(x) (0<×<4),SiO₂ and Al₂O₃. If needed, the thickness of the gate insulating layermay be appropriately controlled by known processes.

According to another example embodiment of the present invention, a gateinsulator containing a crosslinking agent or an organic-inorganic hybridinsulator may be used. The gate insulating layer may have a thicknessfrom about 3,000 Å to about 7,000 Å (e.g., 3,500; 4,000; 4,500; 5,000;5,500; 6,000; or 6,500 Å).

Pursuant to an example embodiment of the present invention, the gateinsulating layer may be formed by, for example, vacuum deposition and/orsolution processing. Other suitable methods for forming the gateinsulating layer may be used.

If necessary, the gate insulating layer may be soft-baked at atemperature from about 60° C. to about 150° C. for a time from about 1to about 10 minutes. Or, according to another example embodiment of thepresent invention, the gate insulating layer may be hard-baked for atemperature from about 100° C. to about 200° C. for a time from about 30minutes to about 3 hours.

Source and drain electrodes may be formed on the gate insulating layer.

Specifically, a metal oxide may be coated on the gate insulating layerby common thin film formation techniques to form a metal oxide thinfilm. Thereafter, the source and drain electrodes may be formed byphotolithography. In particular, the metal oxide thin film may bedeveloped by exposing selected areas (using a photoresist) where sourceand drain electrodes are to be formed (or areas other than the sourceand drain electrodes) to light. Etching may then be carried out usingacetonitrile, etc., by common techniques. The photoresist may then beremoved using a photoresist stripper to form metal oxide source/drainelectrodes on the gate insulating layer.

Examples of suitable metal oxides as materials for the source and drainelectrodes of an organic thin film transistor made according to anexample embodiment of the present invention include, but are not limitedto, indium-tin oxide (ITO) and indium-zinc oxide (IZO).

Deposition of the metal oxide on the gate insulating film may beperformed by, without limitation, thermal evaporation, spin coating,roll coating, spray coating, printing, and the like. Other suitableprocesses for forming a thin film of metal oxide on the gate insulatinglayer may be used.

The metal oxide source/drain electrodes may be surface-treated with aself-assembled monolayer forming compound containing a sulfonic acidgroup under suitable conditions.

The self-assembled monolayer forming compound which may be used for thesurface treatment may be characterized by the presence of a sulfonicacid group (—SO₃H).

The sulfonic acid group acts as an adsorbing group on the surface of,for example, the metal oxide source/drain electrodes. Also, the H⁺ ofthe sulfonic acid group has a strong tendency to dissociate. In otherwords, the group (—SO₃H) readily loses its H⁺ to the metal oxide layerto yield a SO₃ ⁻ group which is then more easily adsorbed to the metaloxide surface.

Examples of self-assembled monolayer forming compounds containing asulfonic acid group that may be used in example embodiments of thepresent invention include, but are not limited to, compounds of Formulae1 to 3 below:

wherein

m, n, a and b are integers satisfying the relations 0<m≦10,000,0≦n<10,000, 0≦a≦20, and 0≦b≦20, respectively,

A, B, A′ and B′ are each independently selected from the groupconsisting of C, Si, Ge, Sn, and Pb,

R₁, R₂, R₃, R₄, R₁′, R₂′, R₃′ and R4′ are each independently selectedfrom the group consisting of hydrogen, halogens, nitro groups,substituted and unsubstituted amino groups, cyano groups, substitutedand unsubstituted C₁-C₃₀ alkyl groups, substituted and unsubstitutedC₁-C₃₀ heteroalkyl groups, substituted and unsubstituted C₁-C₃₀ alkoxygroups, substituted and unsubstituted C₁-C₃₀ heteroalkoxy groups,substituted and unsubstituted C₆-C₃₀ aryl groups, substituted andunsubstituted C₆-C₃₀ arylalkyl groups, substituted and unsubstitutedC₆-C₃₀ aryloxy groups, substituted and unsubstituted C₂-C₃₀ heteroarylgroups, substituted and unsubstituted C₂-C₃₀ heteroarylalkyl groups,substituted and unsubstituted C₂-C₃₀ heteroaryloxy groups, substitutedand unsubstituted C₅-C₂₀ cycloalkyl groups, substituted andunsubstituted C₂-C₃₀ heterocycloalkyl groups, substituted andunsubstituted C₁-C₃₀ alkylester groups, substituted and unsubstitutedC₁-C₃₀ heteroalkylester groups, substituted and unsubstituted C₆-C₃₀arylester groups, and substituted and unsubstituted C₂-C₃₀heteroarylester groups, with the proviso that at least one of R₁, R₂, R₃and R₄ contains a sulfonic acid group, andX and X′ are each independently selected from the group consisting of asingle bond, O, N, S, substituted and unsubstituted C₁-C₃₀ alkylenegroups, substituted and unsubstituted C₁-C₃₀ heteroalkylene groups,substituted and unsubstituted C₆-C₃₀ arylene groups, substituted andunsubstituted C₆-C₃₀ arylalkylene groups, substituted and unsubstitutedC₂-C₃₀ heteroarylene groups, substituted and unsubstituted C₂-C₃₀heteroarylalkylene groups, substituted and unsubstituted C₅-C₂₀cycloalkylene groups, substituted and unsubstituted C₅-C₃₀heterocycloalkylene groups, substituted and unsubstituted C6-C30arylester groups, and substituted and unsubstituted C₂-C₃₀heteroarylester groups;

wherein

o is an integer from 1 to about 10,000, and p is an integer from 0 toabout 10,000, and

X₁, X₂, X₃, Y₁ and Y₂ are each independently hydrogen, fluorine, aC₆-C₃₀ aromatic group, or a C₅-C₃₀ heteroaromatic group interrupted byat least one hetero atom in which the aromatic and heteroaromatic groupsmay be substituted with at least one group selected from C₁-C₁₂ alkylgroups, alkoxy groups, ester groups, carboxylic groups, thiol groups andamine groups;

wherein

1 is an integer from 1 to about 5, and

Z is hydrogen, fluorine, a C₆-C₃₀ aromatic group, or a C₅-C₃₀heteroaromatic group in which the aromatic and heteroaromatic groups maybe substituted with at least one group selected from C₁-C₁₂ alkylgroups, alkoxy groups, ester groups, carboxylic groups, thiol groups andamine groups.

Specific examples of the substituent “alkyl group” as used hereininclude linear and branched methyl, ethyl, propyl, isobutyl, sec-butyl,tert-butyl, pentyl, iso-amyl, hexyl, and the like. At least one hydrogenatom included in the alkyl group may be substituted with a halogen atom,a hydroxyl group, a nitro group, a cyano group, an amino group (—NH₂,—NH(R) or —N(R′)(R″), where R′ and R″ are each independently a C₁-C₁₀alkyl group), an amidino group, a hydrazine group, or a hydrazone group.

The substituent “heteroalkyl group” as used herein refers to an alkylgroup in which one or more carbon atoms, for example, one to five carbonatoms, in the main alkyl chain are substituted with at least one heteroatom, e.g., oxygen, sulfur, nitrogen and phosphorous atoms.

The substituent “aryl group” as used herein refers to a carbocyclicaromatic system including at least one aromatic ring in which the ringsare bonded or fused together in a pendant manner. Specific non-limitingexamples of the aryl group include aromatic groups, e.g., phenyl,naphthyl, and tetrahydronaphthyl. At least one hydrogen atom included inthe aryl group may be substituted with the same substituent as definedwith respect to the substituent “alkyl group.”

The substituent “heteroaryl group” as used herein refers to a C₅-C₃₀cyclic aromatic system in which one to three hetero atoms selected fromN, O, P and S are included, the remaining ring atoms are carbon, and therings are bonded or fused together in a pendant manner. At least onehydrogen atom included in the heteroaryl group may be substituted withthe same substituent as defined with respect to the substituent “alkylgroup.”

The substituent “alkoxy group” as used herein represents alkyl-O-radicalwhere alkyl is as defined above. Specific non-limiting examples of thealkoxy group include methoxy, ethoxy, propoxy, isobutyloxy,sec-butyloxy, pentyloxy, iso-amyloxy, and hexyloxy. At least onehydrogen atom included in the alkoxy group may be substituted with thesame substituent as defined with respect to the substituent “alkylgroup.”

The substituent “heteroalkoxy group” as used herein has substantiallythe same meaning as the substituent “alkoxy group,” except that at leastone hetero atom, for example, oxygen, sulfur or nitrogen, is present inthe alkyl chain. As non-limiting examples of heteroalkoxy groups, theremay be mentioned CH₃CH₂OCH₂CH₂O—, C₄H₉OCH₂CH₂OCH₂CH₂O— and CH₃O(CH₂CH₂O)_(n)H.

The substituent “arylalkyl group” as used herein refers to a substituentin which hydrogen atoms included in the aryl group defined above arepartly substituted with lower alkyl groups, such as methyl, ethyl andpropyl. Non-limiting examples of the arylalkyl group include benzyl andphenylethyl. At least one hydrogen atom included in the arylalkyl groupmay be substituted with the same substituent as defined with respect tothe substituent “alkyl group.”

The substituent “heteroarylalkyl group” as used herein refers to asubstituent in which hydrogen atoms included in the heteroaryl groupdefined above are partly substituted with lower alkyl groups, such asmethyl, ethyl and propyl. Non-limiting examples of the arylalkyl groupinclude benzyl and phenylethyl. At least one hydrogen atom included inthe heteroarylalkyl group may be substituted with the same substituentas defined with respect to the substituent “alkyl group.”

The substituent “aryloxy group” as used herein represents anaryl-O-radical wherein aryl is as defined above. Specific non-limitingexamples of the aryloxy group include phenoxy, naphthoxy,anthracenyloxy, phenanthrenyloxy, fluorenyloxy, and indenyloxy. At leastone hydrogen atom included in the aryloxy group may be substituted withthe same substituent as defined with respect to the substituent “alkylgroup.”

The substituent “heteroaryloxy group” as used herein represents aheteroaryl-O-radical wherein heteroaryl is as defined above. Specificnon-limiting examples of the heteroaryloxy group include benzyloxy andphenylethyloxy groups. At least one hydrogen atom included in theheteroaryloxy group may be substituted with the same substituent asdefined with respect to the substituent “alkyl group.”

The substituent “cycloalkyl group” as used herein refers to a monovalentmonocyclic system having from about 5 to about 30 carbon atoms. At leastone hydrogen atom included in the cycloalkyl group may be substitutedwith the same substituent as defined with respect to the substituent“alkyl group.”

The substituent “heterocycloalkyl group” as used herein refers to aC₅-C₃₀ monovalent monocyclic system in which one to three hetero atomsselected from N, O, P and S are included, and the remaining ring atomsare carbon. At least one hydrogen atom included in the heterocycloalkylgroup may be substituted with the same substituent as defined withrespect to the substituent “alkyl group.”

The substituent “alkylester group” as used herein refers to a functionalgroup in which an alkyl group is bonded to an ester group. The alkylgroup is as defined above.

The substituent “heteroalkylester group” as used herein refers to afunctional group in which a heteroalkyl group is bonded to an estergroup. The heteroalkyl group is as defined above.

The substituent “arylester group” as used herein refers to a functionalgroup in which an aryl group is bonded to an ester group. The aryl groupis as defined above.

The substituent “heteroarylester group” as used herein refers to afunctional group in which a heteroaryl group is bonded to an estergroup. The heteroaryl group is as defined above.

The substituent “amino group” as used herein refers to —NH₂,—NH(R) or—N(R′)(R″) where R′ and R″ are each independently a C₁-C₁₀ alkyl group.

At least one hydrogen atom included in the above-mentioned substituentsmay be substituted with a halogen atom. According to an exampleembodiment of the present invention, the halogen atom maybe a fluorineatom.

According to an example embodiment of the present invention, theself-assembled monolayer forming compounds containing a sulfonic acidgroup, represented by Formulae 1 to 3, may contain a fluorine atom.

In an example embodiment of the present invention, when the metal oxidesource/drain electrodes are surface-treated with the self-assembledmonolayer forming compound containing a fluorine atom, the fluorine atomacts as an electron acceptor to attract electrons from an organicsemiconductor material which may constitute the organic semiconductorlayer, causing hole-doping effects. As a result, according to an exampleembodiment of the present invention, the metal oxide constituting thesource/drain electrodes may have a higher work function, hydrophobicquality and/or charge carrier mobility than that of the organicsemiconductor material itself.

Among self-assembled monolayer forming compounds containing a sulfonicacid group that can be represented by Formula 1, non-limiting examplesof compounds containing a fluorine atom can be represented by thefollowing Formulae 4 to 7:

wherein q is an integer of from 1 to about 10,000, and x and y are eachindependently an integer of from 0 to about 10;

wherein r and s are integers satisfying the relations 0<r≦10,000 and0≦s<10,000, respectively, and w and z are each independently an integerof from 0 to about 20;

wherein t and u are integers satisfying the relations 0<t≦10,000 and0≦u<10,000, respectively, and v and c are each independently an integerof from 0 to about 20;

wherein d and e are integers satisfying the relations 0<d≦10,000 and0≦e<10,000, respectively, and

wherein f is an integer from 0 to about 20.

According to an example embodiment of the present invention, amongself-assembled monolayer forming compounds containing a sulfonic acidgroup that may be represented by Formula 2, compounds containing afluorine atom include, but are not limited to, CF₃(CF₂)_(j)SO₃H,CF₃(CH₂)jSO₃H, CF₃(CF₂)_(j)(CH₂)_(k)SO₃H, and CH₃(CF₂)_(j)(CH₂)_(k)SO₃H(where j and k are each independently an integer of from 1 to about 20).

Pursuant to another example embodiment of the present invention, amongself-assembled monolayer forming compounds that may be represented byFormula 3, compounds containing a fluorine atom can be represented,without limitation, by either of the following Formulae 8 and 9:

wherein i is an integer of from 1 to about 4;

wherein h is an integer of from 1 to about 5.

In accordance with another example embodiment of the present invention,the self-assembled monolayer forming compounds (used in the variousexample embodiments of the present invention) may have a preference forbinding to the surface of the metal oxide as compared to binding to thesurface of the gate insulating layer. Thus, in accordance with anexample embodiment of the present invention, when the gate insulatinglayer on which the metal oxide source/drain electrodes are formed issurface-treated with the self-assembled monolayer forming compounds,these compounds are strongly (and selectively, or substantiallyselectively) bound to the surface of the metal oxide source/drainelectrodes, so that the current transfer characteristics (e.g., chargecarrier mobility), hydrophobic quality and/or work function of thesource/drain electrodes may be improved.

According to an example embodiment of the present invention, the surfacetreatment of the metal oxide source/drain electrodes with theself-assembled monolayer compound containing a sulfonic acid group maybe performed using a solution of the self-assembled monolayer formingcompound in a solvent selected from the group consisting of water,organic solvents and mixtures thereof. Other suitable solvents may beused. More specifically, in an example embodiment of the presentinvention, the formation of a self-assembled monolayer may be performedby impregnating the surface of the metal oxide source/drain electrodeswith the self-assembled monolayer forming solution at a particulartemperature for a particular period of time.

In accordance with an example embodiment of the present invention, asolvent used to prepare the self-assembled monolayer forming solution isselected from the group consisting of water, organic solvents andmixtures thereof. For example, the solvent may be a mixed solvent ofwater and at least one organic solvent.

Examples of suitable organic solvents that can be used in exampleembodiments of the present invention include, but are not limited to:alcohols, such as ethanol; ethers; chlorinated alkanes; aromaticsolvents; and glycols.

According to another example embodiment of the present invention, theself-assembled monolayer forming solution (used in an example embodimentof the method of the present invention) may contain from about 0.001 toabout 20 wt % of the self-assembled monolayer forming compoundcontaining a sulfonic acid group, based on a total weight of thesolution.

According to another example embodiment of the present invention, thesurface treatment of the metal oxide source/drain electrodes with theself-assembled monolayer forming solution may be performed at atemperature from about 10 to about 150° C. (e.g., 15, 20, 25, 30, 35,40, 45, 50, 60, 70, 80, 90, 100, 110, 120, 130, or 140° C.) for a timefrom about 10 minutes to about one hour.

An organic semiconductor material may be deposited on the self-assembledmonolayer and the gate insulating layer by common coating techniques toform an organic semiconductor layer.

According to another example embodiment of the present invention,examples of materials suitable for the organic semiconductor layer ofthe organic thin film transistor include, but are not limited to,pentacenes, tetracenes, copper phthalocyanines, polythiophenes,polyanilines, polyacetylenes, polypyrroles, polyphenylene vinylenes, andderivatives thereof. Other suitable organic semiconductor layermaterials may be used. Depositing a coating of the organic semiconductormaterial may be carried out by thermal evaporation, screen printing,printing, spin coating, dip coating, or ink spraying. Other suitablecoating processes may be used.

A method according to an example embodiment of the present invention mayfurther include acid-treating or UV-ozonating the surface of thesource/drain electrodes, prior to surface treatment of the source/drainelectrodes with the self-assembled monolayer forming compound containinga sulfonic acid group.

Such acid treatment or UV ozonation before the application and formationof the self-assembled monolayer on the surface of the source/drainelectrodes may render the surface of the metal oxide more hydrophilic,which may increase the adsorption of the sulfonic acid group to themetal oxide surface.

According to another example embodiment of the present invention, theacid treatment may be performed by impregnating the surface of the metaloxide source/drain electrodes with an organic or inorganic acid solutionat a temperature from about 15 to about 35° C. for a time from about 0.5to about 10 seconds.

An example of organic acid that may be used in an example embodiment ofthe present invention may be represented by Formula 10 below:RCOOH   Formula 10wherein R is a C₁₋₁₂ alkyl, alkenyl or alkynyl group; a C₃₋₃₀ cycloalkylgroup; a C₆₋₃₀ aryl group; or a fluoro-substituted functional group.

Specific examples of inorganic acids that may be used in accordance withan example embodiment of the present invention include, but are notlimited to, HI HBr, HCl, HF, HNO₃, H₃PO₄, H₂SO₄, and mixtures thereof.Mixed acids may also be used according to another example embodiment ofthe present invention. For example, HCl and HNO₃ may be used as a mixedacid for etching, for example, ITO source/drain electrodes.

Also, the UV ozonation may be performed by irradiating the surface ofthe metal oxide source/drain electrodes using a lamp having a power ofabout 0.28 W/cm³ at a wavelength of about 254 nm for a time from about 1to about 30 minutes.

A method for fabricating an organic thin film transistor according to anexample embodiment of the present invention may further includeannealing the source/drain electrodes surface treated with theself-assembled monolayer forming compound.

This annealing may increase the adhesion of the self-assembled monolayerto the source/drain electrodes, enabling the fabrication of transistorshaving a higher charge carrier mobility than that of transistorsfabricated without annealing.

According to an example embodiment of the present invention, theannealing may be performed at a temperature from about 50 to about 200°C. (e.g., 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180,or 190° C., respectively) for a time from about 10 minutes to about onehour.

A method for producing an organic thin film transistor according toanother example embodiment of the present invention may further includeacid-treating or UV-ozonating the surface of the metal oxidesource/drain electrodes, prior to the surface treatment with theself-assembled monolayer forming compound containing a sulfonic acidgroup, and annealing the surface-treated the metal oxide source/drainelectrodes.

Pursuant to an example embodiment of the present invention, an organicthin film transistor fabricated by a method (in accordance with afurther example embodiment) of the present invention may have abottom-contact structure. The structure of such a bottom-contact organicthin film transistor may be as shown in FIG. 1. As shown in the exampleembodiment of the present invention of FIG. 1, the bottom-contactorganic thin film transistor may have a structure including a substrate1, a gate electrode 2, a gate insulating film 3, metal oxidesource/drain electrodes 4 and 5, a self-assembled monolayer 6, and/or anorganic semiconductor layer 7.

According to another example embodiment of the present invention, thesurface of the source/drain electrodes of the organic thin filmtransistor may be changed to be more hydrophobic. By doing so, the metaloxide constituting the source/drain electrodes may have a higher workfunction than the organic semiconductor material. Accordingly, theorganic thin film transistor may show superior electrical properties,particularly, a higher charge carrier mobility.

The organic thin film transistor fabricated by an example embodiment ofthe present invention may be utilized in the manufacture of displaydevices, such as, for example, electroluminescence devices, liquidcrystal devices, and electrophoresis devices.

Additional example embodiments of the present invention will now bedescribed in more detail with reference to the following non-limitingexamples. These example embodiments of the present invention areprovided for the purpose of illustration and are not to be construed aslimiting the scope of the invention.

EXAMPLE 1

Al was deposited on a clean glass substrate by a sputtering technique toform a gate electrode having a thickness of 1,500 Å. Polyvinylphenolcontaining a crosslinking agent was spin-coated thereon at 1,000 rpm toa thickness of 5,000 Å, and soft-baked at 100° C. for 5 minutes to forma gate insulating layer. ITO was deposited on the insulating layer to athickness of 1,000 Å by a thermal evaporation method, and was thensubjected to photolithography to form an ITO electrode pattern. At thistime, the deposition was conducted under a vacuum pressure of 2×10⁻⁷torr, a substrate temperature of 50° C. and at a deposition rate of 0.85Å/sec. Subsequently, the ITO electrode was surface-treated byimpregnating it with a self-assembled monolayer forming solution at roomtemperature for 30 minutes. The self-assembled monolayer formingsolution was prepared by dissolving 5 wt % (based on a total weight ofthe solution) of a perfluorinated resin solution (Nafion®, Aldrich) in amixed solvent (55 wt % ethanol and 45 wt % water). That is, theself-assembled monolayer forming solution consisted of 5 wt % Nafion and95% wt % mixed solvent, and the mixed solvent consisted of 55 wt %ethanol and 45 wt % water. The perfluorinated resin used herein isrepresented by the following Formula 11:

wherein m is an integer of from 5 to 11, and n is 1.

Pentacene was deposited on the self-assembled monolayer and on theinsulating layer to a thickness of 1,000 Å by a thermal evaporationmethod under a vacuum pressure of 2×10⁻⁷ torr, a substrate temperatureof 50° C. and at a deposition rate of 0.85 Å/sec.

EXAMPLE 2

An organic thin film transistor was fabricated in the same manner as inExample 1, except that the surface of the ITO electrode was acid-treatedwith an ITO etchant (HCl—16.7-20.3% in water) at room temperature for 10seconds.

EXAMPLE 3

An organic thin film transistor was fabricated in the same manner as inExample 1, except that the surface of the ITO electrode was subjected toUV ozonation using a lamp having a power of 0.28 W/cm³ at a wavelengthof 254 nm for 3 minutes, prior to the surface treatment of the ITOelectrode.

EXAMPLE 4

An organic thin film transistor was fabricated in the same manner as inExample 1, except that annealing was performed at 200° C. for 30 minutesafter the surface treatment of the ITO electrode but before depositionof the pentacene.

EXAMPLE 5

An organic thin film transistor was fabricated in the same manner as inExample 2, except that the acid-treated ITO electrode wassurface-treated by impregnating with a self-assembled monolayer formingsolution containing 5 wt % (based on a total weight of the solution) ofa nonaperfluorosulfonic acid (C₄F₉SO₃H) solution in a mixed solvent (55wt % ethanol and 45 wt % water) at room temperature for 30 minutes, andthen a solution of 1 wt % (based on a total weight of the toluenesolution) of polythiophene in toluene was applied to a thickness of1,000 Å. As noted above, the self-assembled monolayer forming solutionwas composed of 5 wt % nonaperfluorosulfonic acid (C₄F₉SO₃H) solutionand 95 wt % mixed solvent, and the mixed solvent was composed of 55 wt %ethanol and 45 wt % water.

COMPARATIVE EXAMPLE 1

An organic thin film transistor was fabricated in the same manner as inExample 1, except that the ITO electrode was not surface-treated with aself-assembled monolayer forming compound.

COMPARATIVE EXAMPLE 2

An organic thin film transistor was fabricated in the same manner as inExample 2, except that the ITO electrode was not surface-treated with aself-assembled monolayer forming compound.

COMPARATIVE EXAMPLE 3

An organic thin film transistor was fabricated in the same manner as inExample 3, except that the ITO electrode was not surface-treated with aself-assembled monolayer forming compound.

COMPARATIVE EXAMPLE 4

An organic thin film transistor was fabricated in the same manner as inExample 4, except that the ITO electrode was not surface-treated with aself-assembled monolayer forming compound.

COMPARATIVE EXAMPLE 5

An organic thin film transistor was fabricated in the same manner as inExample 5, except that the ITO electrode was not surface-treated with aself-assembled monolayer forming compound.

The changes in the characteristics of the source/drain electrodessurface-treated with the self-assembled monolayer forming compound andthe gate insulating films were evaluated. Specifically, the contactangle between the metal oxide source/drain electrodes and the gateinsulating film in each of the organic thin film transistors fabricatedin Examples 1 to 4 and Comparative Examples 1 to 4 was determined byadvancing angle measurement using a single drop of distilled water. Inaddition, the work function of the gate insulating films in the organicthin film transistors fabricated in Examples 1 to 4 and ComparativeExamples 1 to 4 was determined by photoelectron emission measurementusing UV rays. The results obtained are shown in Table 1. TABLE 1Contact angle Work function Gate (source/ Source/drain insulating drainExample No. electrode film electrode) Example 1 72.5 55.8 5.15 Example 278.5 53.1 5.56 Example 3 80.5 49.5 5.22 Example 4 68.6 — — ComparativeExample 1 54.8 60.5 5.06 Comparative Example 2 31.1 53.8 4.98Comparative Example 3 28.3 53.3 — Comparative Example 4 56.7 — —

As apparent from the results of Table 1 because the self-assembledmonolayer forming compound for the surface treatment in exampleembodiments of the present invention is selectively bound to the surfaceof the metal oxide source/drain electrodes while not binding (orsubstantially not binding) to the gate insulating layer, the surface ofthe source/drain electrodes becomes more hydrophobic, and the workfunction of the source/drain electrodes is increased, for example, toabove that of the organic semiconductor material (pentacene: 5.1 eV).

The current transfer characteristics of the organic thin filmtransistors fabricated in Examples 1-4 and Comparative Examples 1-4 wereevaluated using a KEITHLEY semiconductor analyzer (4200-SCS), and theobtained curves were plotted in FIG. 2.

To evaluate the electrical properties of the organic thin filmtransistors, the charge carrier mobility of the organic thin filmtransistors fabricated in Examples 1-5 and Comparative Examples 1-5 wascalculated from the slope of a graph representing the relationshipbetween (ISD)^(1/2) and V_(G) from the following current equations inthe saturation region:$I_{SD} = {\frac{W\quad C_{0}}{2L}{\mu\left( {V_{G} - V_{T}} \right)}^{2}}$$\sqrt{I_{SD}} = {\sqrt{\frac{\mu\quad C_{0}W}{2L}}\left( {V_{G} - V_{T}} \right)}$${slope} = \sqrt{\frac{\mu\quad C_{0}W}{2L}}$$\mu_{FET} = {({slope})^{2}\frac{2L}{C_{0}W}}$

In the above equations, ISD=source-drain current, μ and μ_(FET)=chargecarrier mobility, C_(o)=capacitance of the oxide film, W=channel width,L=channel length; V_(G)=gate voltage, and V_(T)=threshold voltage. TABLE2 Example No. Charge carrier mobility (cm²/Vs) Example 1 0.225 Example 21.00 Example 3 0.743 Example 4 0.360 Example 5 0.085 Comparative Example1 0.035 Comparative Example 2 0.183 Comparative Example 3 0.173Comparative Example 4 0.085 Comparative Example 5 0.029

As can be seen from the data shown in Table 2, the organic thin filmtransistors fabricated by an example embodiment of the present inventionshow substantially higher charge carrier mobilities in relation to thoseof Comparative Examples 1-5.

Although example embodiments of the present invention have beendisclosed for illustrative purposes, it should be understood that thescope of the present invention is not limited by these exampleembodiments in any manner and those skilled in the art will appreciatethat various modifications are possible without departing from thetechnical scope and spirit of the invention.

As apparent from the foregoing, according to example embodiments of thepresent invention, the surface of source/drain electrodes may be changedto be more hydrophobic as well as the work function of a metal oxideconstituting the source/drain electrodes may be increased to above thatof an organic semiconductor material. Therefore, an example embodimentof the present invention enables the fabrication of organic thin filmtransistors with higher charge carrier mobility.

1. A method for fabricating an organic thin film transistor including asubstrate, a gate electrode, a gate insulating layer, metal oxidesource/drain electrodes and an organic semiconductor layer, the methodcomprising: treating at least one surface of the metal oxidesource/drain electrodes with a self-assembled monolayer (SAM) formingcompound containing a sulfonic acid group.
 2. The method according toclaim 1, wherein the self-assembled monolayer forming compoundcontaining a sulfonic acid group is represented by any one of Formulae 1to 3 below:

wherein m, n, a and b are integers satisfying the relations 0<m≦10,000,0≦n<10,000, 0≦a≦20, and 0≦b≦20, respectively, A, B, A′ and B′ are eachindependently selected from the group consisting of C, Si, Ge, Sn, andPb, R₁, R₂, R₃, R₄, R₁′, R₂′, R₃′ and R₄′ are each independentlyselected from the group consisting of a hydrogen, a halogen, a nitrogroup, a substituted or unsubstituted amino group, a cyano group, asubstituted or unsubstituted C₁-C₃₀ alkyl group, a substituted orunsubstituted C₁-C₃₀ heteroalkyl group, a substituted or unsubstitutedC₁-C₃₀ alkoxy group, a substituted or unsubstituted C₁-C₃₀ heteroalkoxygroup, a substituted or unsubstituted C₆-C₃₀ aryl group, a substitutedor unsubstituted C₆-C₃₀ arylalkyl group, a substituted or unsubstitutedC₆-C₃₀ aryloxy group, a substituted or unsubstituted C₂-C₃₀ heteroarylgroup, a substituted or unsubstituted C₂-C₃₀ heteroarylalkyl group, asubstituted or unsubstituted C₂-C₃₀ heteroaryloxy group, a substitutedor unsubstituted C₅-C₂₀ cycloalkyl group, substituted or unsubstitutedC₂-C₃₀ heterocycloalkyl group, a substituted or unsubstituted C₁-C₃₀alkylester group, a substituted or unsubstituted C₁-C₃₀ heteroalkylestergroup, a substituted or unsubstituted C₆-C₃₀ arylester group, and asubstituted or unsubstituted C₂-C₃₀ heteroarylester group, with theproviso that at least one of R₁, R₂, R₃ and R₄ contains a sulfonic acidgroup, and X and X′ are each independently selected from the groupconsisting of a single bond, O, N, S, a substituted or unsubstitutedC₁-C₃₀ alkylene group, a substituted or unsubstituted C₁-C₃₀heteroalkylene group, a substituted or unsubstituted C₆-C₃₀ arylenegroup, a substituted or unsubstituted C₆-C₃₀ arylalkylene group, asubstituted or unsubstituted C₂-C₃₀ heteroarylene group, a substitutedor unsubstituted C₂-C₃₀ heteroarylalkylene group, a substituted orunsubstituted C₅-C₂₀ cycloalkylene group, a substituted or unsubstitutedC₅-C₃₀ heterocycloalkylene group, a substituted or unsubstituted C₆-C₃₀arylester group, and a substituted or unsubstituted C₂-C₃₀heteroarylester group;

wherein o is an integer from 1 to about 10,000, and p is an integer from0 to about 10,000, and X₁, X₂, X₃, Y₁ and Y₂ are each independentlyhydrogen, fluorine, a C₆-C₃₀ aromatic group, or a C₅-C₃₀ heteroaromaticgroup interrupted by at least one hetero atom in which the aromatic andheteroaromatic groups are unsubstituted or substituted with at least onegroup selected from the group consisting of a C₁-C₁₂ alkyl group, analkoxy group, an ester group, a carboxylic group, a thiol group and anamine group;

wherein 1 is an integer from 1 to about 5, and Z is hydrogen, fluorine,a C₆-C₃₀ aromatic group, or a C₅-C₃₀ heteroaromatic group in which thearomatic and heteroaromatic groups may be substituted with at least onegroup selected from the group consisting of a C₁-C₁₂ alkyl group, analkoxy group, an ester group, a carboxylic group, a thiol group and anamine group.
 3. The method according to claim 2, wherein theself-assembled monolayer forming compound containing a sulfonic acidgroup represented by any one of Formulae 1 to 3 contains a fluorineatom.
 4. The method according to claim 1, wherein the self-assembledmonolayer forming compound containing a sulfonic acid group isrepresented by Formula 1 which is further represented by any one ofFormulae 4 to 7 below:

wherein q is an integer from 1 to about 10,000, and x and y are eachindependently an integer from 0 to about 10;

wherein r and s are integers satisfying the relations 0<r≦10,000 and0≦ns<10,000, respectively, and w and z are each independently an integerfrom 0 to about 20;

wherein t and u are integers satisfying the relations 0<t≦10,000 and0≦u<10,000, respectively, and v and c are each independently an integerof from 0 to about 20;

wherein d and e are integers satisfying the relations 0<d≦10,000 and0≦e<10,000, respectively, and wherein f is an integer from 0 to about20.
 5. The method according to claim 3, wherein the self-assembledmonolayer forming compound containing a sulfonic acid group and afluorine atom is represented by Formula 2 which is selected from thegroup consisting of CF₃(CF₂)_(j)SO₃H, CF₃(CH₂)_(j)SO₃H,CF₃(CF₂)_(j)(CH₂)_(k)SO₃H, and CH₃(CF₂)_(j)(CH₂)_(k)SO₃H, wherein j andk are each independently an integer of from 1 to about
 20. 6. The methodaccording to claim 3, wherein the self-assembled monolayer formingcompound containing a sulfonic acid group and a fluorine atom isrepresented by Formula 3 which is further represented by either ofFormulae 8 or 9:

wherein i is an integer from 1 to about 4;

wherein h is an integer from 1 to about
 5. 7. The method according toclaim 1, wherein treating at least one surface of the metal oxidesource/drain electrodes with a self-assembled monolayer forming compoundcontaining a sulfonic acid group is performed using a solution of theself-assembled monolayer forming compound in a solvent selected from thegroup consisting of water, organic solvents and mixtures thereof.
 8. Themethod according to claim 7, wherein the solution of the self-assembledmonolayer forming contains from about 0.001 to about 20 wt % of theself-assembled monolayer forming compound containing a sulfonic acidgroup based on a total weight of the solution.
 9. The method accordingto claim 7, wherein the organic solvent is selected from the groupconsisting of an alcohol, ethanol, an ether, a chlorinated alkane, anaromatic solvent, a glycol, and mixtures thereof.
 10. The methodaccording to claim 7, wherein treating at least one surface of the metaloxide source/drain electrodes with the self-assembled monolayer formingsolution is performed at a temperature from about 10 to about 150° C.for a time from about 10 minutes to about 1 hour.
 11. The methodaccording to claim 1, further including: acid-treating or UV-ozonatingthe at least one surface of the source/drain electrodes prior totreating the at least one surface of the source/drain electrodes withthe self-assembled monolayer forming compound containing a sulfonic acidgroup.
 12. The method according to claim 11, wherein the acid treatmentis performed by impregnating the at least one surface of the metal oxidesource/drain electrodes with an organic or inorganic acid solution at atemperature from about 15 to about 35° C. for a time from about 0.5 toabout 10 seconds.
 13. The method according to claim 11, wherein the UVozonation is performed by irradiating the at least one surface of themetal oxide source/drain electrodes using a lamp having a power of about0.28 W/cm³ at a wavelength of about 254 nm for about 1 to about 30minutes.
 14. The method according to claim 1, further including:annealing after treating the at least one surface of the source/drainelectrodes with the self-assembled monolayer forming compound.
 15. Themethod according to claim 11, further including: annealing aftertreating the at least one surface of the source/drain electrodes withthe self-assembled monolayer forming compound containing a sulfonic acidgroup.
 16. The method according to claim 14, wherein the annealing isperformed at a temperature from about 50 to about 200° C. for a timefrom about 10 minutes to about one hour.
 17. The method according toclaim 15, wherein the annealing is performed at a temperature from about50 to about 200° C. for a time from about 10 minutes to about one hour.18. The method according to claim 1, wherein the substrate is made of amaterial selected from the group consisting of glass, silicon andplastic.
 19. The method according to claim 1, wherein the gate electrodeis made of a material selected from the group consisting of gold (Au),silver (Ag), aluminum (Al), nickel (Ni), molybdenum (Mo), tungsten (W),indium-tin oxides (ITO), polythiophenes, polyanilines, polyacetylenes,polypyrroles, polyphenylene vinylenes, and polyethylenedioxythiophene(PEDOT)/polystyrenesulfonate (PSS) mixtures.
 20. The method according toclaim 1, wherein the gate insulating layer is made of a materialselected from the group consisting of organic materials, polyolefins,polyvinyls, polyacrylates, polystyrenes, polyurethanes, polyimides,polyvinylphenols and derivatives thereof, and inorganic materials, andSiN_(x) (0<×<4), SiO₂ and Al₂O₃.
 21. The method according to claim 1,wherein the metal oxide source/drain electrodes are made of a materialselected from the group consisting of indium-tin oxide (ITO) andindium-zinc oxide (IZO).
 22. The method according to claim 1, whereinthe organic semiconductor layer is made of a material selected from thegroup consisting of pentacenes, tetracenes, copper phthalocyanines,polythiophenes, polyanilines, polyacetylenes, polypyrroles,polyphenylene vinylenes, and derivatives thereof.
 23. An organic thinfilm transistor fabricated by the method according to claim
 1. 24. Anorganic thin film transistor fabricated by the method of claim
 11. 25.An organic thin film transistor fabricated by the method of claim 14.26. An organic thin film transistor fabricated by the method of claim15.
 27. A display device comprising the organic thin film transistor ofclaim
 24. 28. A display device comprising the organic thin filmtransistor of claim
 25. 29. A display device comprising the organic thinfilm transistor of claim 26.